A modem is a communication device which couples a computer to a telephone line to enable the computer to communicate with other computers. In recent years, faxmodems have been developed to enable their host computers to receive and transmit both facsimile and computer modem data over a single telephone line. Since data modems and facsimile machines or faxmodems use different data formats, the host must be able to identify an incoming call as either a facsimile type call or a data call so that it can process the incoming call appropriately.
The vast majority of facsimile machines presently in use transmit a calling tone to establish communication with a receiving device before the facsimile data is sent. Typically, data modems do not transmit a calling tone. So, a simple way to identify a caller as either facsimile or data is to wait for the facsimile calling tone. If it is not received within a predefined time frame, then it is assumed that the call is a data call and the receiving modem is configured to receive data. If the fax calling tone is received, the modem is configured to receive facsimile data.
A problem arises when it is desired to also process voice telephone calls on the same telephone line. Voice callers, like most data modems, do not send calling tones. So, it is difficult for a receiving device such as a modem to distinguish between a voice call and a data call. Also, if the receiving device answers the call and then requires a long period of time to determine what type of call it is, a voice caller may become discouraged and terminate his call.